Add throws section

Add throws section like in Dinero docs:

https://dinerojs.com/module-dinero#~add

Start by reviewing the referenced Dinero add documentation at https://dinerojs.com/module-dinero#~add and locate the corresponding EasyMoney API documentation entry. Add a throws section matching the relevant Dinero documentation style, then verify that the add documentation clearly describes its thrown errors.

Written by the indexing model from the issue text.